Two chemical modification techniques will be developed for the study of peptide receptors in biological membranes: Endo photoaffinity peptide reagents, in which an aromatic residue is replaced with a (4-azido-2-nitro)-L-phenylalanyl residue, will be used to identify peptide receptors in membranes. Membrane-impermeant, cleavable crosslinking reagents will be used to probe the macromolecular environment of a labeled receptor. These two procedures, used in tandem, will be employed in a model study of the E. coli dipeptide transport system: 1) To identify the dipeptide receptor on the extracytoplasmic face of the membrane and 2) To identify other structures intimately associated with the receptor in the plane of the membrane. Some of these techniques will then be utilized in an attempt to identify the E. coli secretory receptor, i.e., the membrane structure which recognizes the initial sequence of the nascent chains of secreted proteins and mediates the transport of the growing polypeptide chains through the membrane. We will also apply these techniques in a study of the receptor for the mitogenic and gastric antisecretory hormone epidermal growth factor in the membrane of A-431 cells.